feat: add duplicate account import prevention #189

Add duplicate check in ImportAccountView before account import
Add duplicate check in ImportDerivedAccountView for derived accounts
Add safety check in saveNewIdentity function to prevent duplicate saves
Implement user-friendly warning messages for duplicate attempts
Add comprehensive error handling to catch duplicate errors from saveNewIdentity
Create Playwright tests to verify duplicate prevention functionality
Add documentation for duplicate prevention implementation

The system now prevents users from importing the same account multiple times
by checking for existing DIDs both before import (pre-check) and during
save (post-check). Users receive clear warning messages instead of
technical errors when attempting to import duplicate accounts.

Files modified:

src/views/ImportAccountView.vue: Add duplicate check and error handling
src/views/ImportDerivedAccountView.vue: Add duplicate check for derived accounts
src/libs/util.ts: Add duplicate prevention in saveNewIdentity
test-playwright/duplicate-import-test.spec.ts: Add comprehensive tests
doc/duplicate-account-import-implementation.md: Add implementation docs

Resolves: Prevent duplicate account imports in IdentitySwitcherView

@trentlarson @anomalist I wanted to hear your thoughts on the changes, specifically the redundancies with the duplication checks when deriving from an existing seed and when adding a new identity. Is it too much, or just right?

This all looks good. I just tweaked the error-message logic, so you can see that they are guaranteed to use the exact same message.

I just added another commit for a small change: I removed the "catch" from a couple places because all it did was log a message and then rethrow the error, while adding some logging. (This can get confusing, especially when the calling code knows how to handle the error, so I prefer just to let it propagate.)

Duly noted on those last changes.